This invention relates to a demodulator operable in response to either of a stereophonic signal and a monophonic or monaural signal to demodulate one of the stereophonic and the monophonic signals into a demodulated or audio signal. Each of the stereophonic and the monophonic signals is subjected to frequency modulation. In particular, the stereophonic signal is given in the form of a composite signal comprising, in addition to a pilot signal of a predetermined frequency, for example, 19 kHz, a main channel and a subchannel signal carrying a right and a left signal.
As will later be described with reference to a few figures of the accompanying drawing, a conventional demodulator of the type described is supplied as an input signal with either a stereophonic signal or a monophonic signal and comprises a phase-locked loop circuit for producing a frequency signal necessary for demodulation of the input signal and a decoder for decoding the input signal into the audio signal with reference to the frequency signal. When the stereophonic signal is received as the input signal, the pilot signal should be cancelled from the audio signal. For this purpose, a pilot signal cancellation circuit is coupled between the phase-locked loop circuit and the decoder.
Furthermore, an indicator is included in the demodulator to indicate reception of the stereophonic signal. In general, the indicator is lit or illuminated when the pilot signal is detected from the stereophonic signal and, otherwise, the indicator is darkened. In the conventional demodulator, the indicator is driven by an indicator drive circuit independent of the phase-locked loop, the pilot signal cancellation circuit, and the decoder. This means that the conventional demodulator is intricate in structure because of presence of a superfluous circuit, such as the indicator drive circuit.
In addition, it has been a recent trend that the demodulator is mounted in a wide variety of audio receivers, such as a high fidelity audio receiver, a portable audio receiver, an audio receiver on vehicle. As a rule, the indicator for each radio receiver is lit by the use of the indicator drive circuit by a driving signal of a level peculiar to each radio receiver. As a result, the indicator drive circuits should be changed from one another for various uses. This is because the driving signal level can not be changed in the conventional demodulator.